1. Field of the Invention
The present invention relates to an induction heating device and an induction heat fixing device, which is mounted in an image forming apparatus, such as a copier, a printer or a facsimile, includes a heating target member which is heated by induction heating, and fixes a toner image on a sheet by the heating target member.
2. Description of the Related Art
As a fixing device used for an image forming apparatus such as an electro photographic copier or printer, there is a device in which a sheet paper is inserted into a nip formed between a pair of rollers including a heat roller and a press roller or between similar belts, and a toner image is heated, pressed and fixed. In such a heating type fixing device, in order to realize speed-up of process speed, there is an induction heat fixing device in which a heat roller or a heating belt is heated by an induction heating system.
As one of such induction heat fixing devices, there is a device in which an induction current generating coil is disposed outside a heat roller having a metal conductive layer or a heating belt to be opposite thereto. In the device as stated above, an eddy-current is generated in the metal conductive layer by a magnetic field generated by supplying a specified power to the induction current generating coil. The metal conductive layer is instantaneously heated by this eddy-current, and for example, heating of the heat roller is performed. Further, in the fixing device of the induction heating system as stated above, in order to uniform the temperature distribution of the heat roller in the longitudinal direction, there is a device in which an induction current generating coil is divided into plural parts.
However, when the induction current generating coil is divided into plural parts as stated above, magnetic field at a joint portion between adjacent induction current generating coils is reduced. Thus, the temperature of the heat roller is lowered at the joint portion between the adjacent induction current generating coils, and there is a fear that temperature unevenness occurs in the heat roller. For example, as shown in the Prior Art of FIG. 1, in the case where induction current generating coils 102 and 103 in each of which a litz wire 101 including 50 enamel wires with a wire diameter of 0.3 mm is wound six turns in a simple shape are adjacent to each other, the coil width of a joint portion (α) becomes 36 mm to 40 mm. As stated above, when the width of the joint portion to cause the reduction of magnetic field is wide, there is a fear that temperature unevenness of the heat roller becomes noticeable.
Thus, for example, JP-A-9-237675 discloses a heating device in which all ends of exciting coils provided in the inside of a heat roller are stacked, and the width of the exciting coil at the end is narrowed.
However, in the device in which the induction current generating coil is divided into the plural parts, at the joint portion between the adjacent induction current generating coils, a mutual induction current is generated between the induction current generating coils. When all coils are stacked in the induction current generating coils, the facing area of the adjacent coils at the joint portion becomes large. As a result, the mutual induction current generated between both the coils of the joint portion becomes large, and there is a fear that temperature unevenness of the heat roller occurs.
Especially, there is a device in which for further speed-up, a thinner and small heat capacity metal conductive layer is provided near the surface, and the metal conductive layer is heated by an external induction current generating coil. In such a device, a mutual induction current generated in a joint portion of induction current generating coils has a remarkable influence on temperature unevenness of the heat roller, and there is a fear that poor fixation occurs.
Then, in a heating device to heat a metal conductive layer by using induction current generating coils divided into plural parts, there is desired an induction heating device in which temperature unevenness due to a joint portion between adjacent induction current generating coils is reduced, and the temperature is made uniform. Besides, there is desired an induction heat fixing device which can obtain excellent fixing properties by using this induction heating device.